The present invention provides a method and system of controlling an electronic brake system for a vehicle. The method includes estimation of the caliper pressure and comparing the estimated pressure to the commanded pressure. The method eliminates the need for downstream caliper pressure sensors and corresponding control methods of the brake system.
Heretofore, downstream caliper pressure sensors have been required in electronically controlled vehicle braking systems, such as the brake-by-wire type system defined in U.S. Pat. No. 5,558,409 entitled xe2x80x9cElectrohydraulic Braking Systemxe2x80x9d, for use in comparing sensed actual caliper pressure feedback from a downstream caliper pressure sensor to commanded or required brake pressure, with any error between the two being accommodated for by varying the position of solenoid operated apply and release valves of the caliper actuation system. These systems typically requires a set of pressure sensors, each one of which is associated with a single wheel brake assembly.
As will be described in greater detail hereinbelow, the method of the present invention eliminates the need for the downstream caliper pressure sensors and associated control thereof, and also shortens error accommodation response time, allowing larger valve commands in transitions, which results in faster response time for the system, thus enhancing braking capability of the system.
According to the invention there is provided a method and system for use in an electronic processor controlled vehicular braking system. The method allows elimination of caliper pressure sensors and any controls systems and associated algorithms from the braking system, while improving braking capability through decreasing response time of the system.
In the brake-by-wire system, like that used with the present invention, it will be understood that a pump applies high-pressure fluid to an accumulator. The accumulator stores a large volume of high-pressure fluid. The accumulator is used so that high-pressure fluid is always available on demand. Without the accumulator, the system would have to wait until the pump generates pressure before the pressure can be used to activate the caliper. A reservoir is used to hold fluid, which can be sent to the low-pressure side of the pump and also collects fluid from a release valve. Brake calipers convert the fluid pressure to force, which is applied to brake rotors. As fluid enters the calipers the caliper pistons are displaced. The amount of fluid to achieve desired pressure is a function of the caliper compliance.
With respect to operation of a brake-by-wire system, it should be understood that three important forces are acting on each valve, also known as a proportional poppet valve. The three forces include a force applied by an electrical coil, a force applied by fluid pressure and a biasing force applied by a spring in the valve. When a current is applied to the coil, the poppet is moved in such a way to permit fluid to flow between the accumulator to the caliper. As the fluid pressure increases in the caliper, the pressure difference across the valve reduces and therefore the fluid pressure force reduces. Thus, when the coil force and the spring force remain unchanged the valve begins to close. The proportional poppet valve will close completely when the differential pressure across the valve is reduced and the spring force overcomes the coil and any remaining fluid pressure force. Therefore, a given coil current results in a given caliper pressure.
Because the object of the present invention is to eliminate the need for caliper pressure sensors, fluid pressure at the caliper is estimated. To estimate the pressure at the caliper, fluid flow through the valve is determined. For a given coil and pressure force the valve will move to a particular position. If the pressure force is reduced or increased the valve will move accordingly and the gap in the valve or opening therethrough will change. If the coil forces increase or decrease the gap will also change accordingly. The gap in the valve determines the fluid flow through the valve. Therefore, for a given current to the coil and a given differential pressure across the valve there will be a given flow through the valve, which can be determined experimentally on a flow bench.
With knowledge of the caliper compliance (determined experimentally) and the fluid flow through the valve, pressure inside the caliper can be estimated. Pressure inside the caliper is a function of the volume of the fluid inside the caliper. The volume of incoming fluid can be determined by integrating the flow through the apply valve. The volume of the fluid leaving the caliper is determined by integrating the flow through the release valve. The volume of fluid in the caliper is known and therefore the caliper pressure is known. With this information, the pressure at the caliper can be estimated, and control of the valves can occur without a pressure sensor located at each caliper.
One aspect of the present invention provides a method of controlling a brake system including eliciting a command pressure value from a brake processor or control module of the system; initializing and generating an estimation of caliper pressure through use of empirically determined system specific data in control module memory, comparing the command pressure value to the estimated pressure value and, if a correlation error exists, calculating opening and closing parameters for apply and release valves for caliper actuation medium to eliminate the error; and cycling back through at least the above steps in closed loop fashion until the comparison of command and estimated pressures provides a substantially no error reading, indicating a steady state between the estimated and required or command caliper pressure.
Through use of a second method of the set, estimated caliper pressure for each cycle is determined, and through use of a third method of the set, the apply and release valves are constantly manipulated about a steady state position for increasing reactivity thereof, and eliminating any steady state error that might exist, over time.
An aspect of the present invention includes a brake control method for use in an electronically controlled brake-by-wire type vehicular braking system including determining a command caliper pressure, determining an estimated caliper pressure, comparing the command to the estimated caliper pressure to produce an error value, multiplying the error value by a predetermined gain to produce a valve command and controlling apply and release valves responsive to the valve command.
In other aspects of the present invention the determination of the command caliper pressure includes determining a command pressure value from a brake control module of the system, determining a wheel speed, calculating a wheel speed gain value from the sensed wheel speed and proportioning the wheel speed gain value to the command pressure value to provide a proportioned wheel speed gain value, the proportioned wheel speed gain value providing the command caliper pressure.
In other aspects of the present invention the determination of the estimated caliper pressure includes sensing a supply pressure reading from a high pressure source for the caliper actuating medium, eliciting a past cycle estimated pressure value, comparing the supply pressure with the estimated past cycle pressure value to obtain a differential pressure reading across the caliper and comparing the differential pressure reading, the past cycle pressure value reading, a past value direction reading and a past valve operation command to corresponding lookup tables for apply pressure vs. valve position and release pressure vs. valve position. The lookup table results can be saturated to assure they are within limits of the system. An estimation of caliper pressure can be initiated from the saturated input parameters when a brake pedal of the system is applied and the estimation can be saturated within system limits. In other aspects of the method of the present invention the command pressure can be compared to the saturated estimated pressure and, if an error exists, appropriate opening and closing of the apply and release valves are controlled to eliminate the error and the steps are cycled through in closed loop fashion until the comparison of command and estimated pressures provides a substantially negligible error reading, indicating a steady state.
The method of the present invention can include name dithering process in the change of command caliper pressure function. The dithering process can be apply to the command caliper pressure when the pressure is at a steady state above a predefined lower limit, such that fluctuations of the dither are applied to the command caliper pressure at steady state to keep apply and release valves of the system constantly operational rather than fixed in position, while still maintaining the steady state.
The rates of flow, through both apply and release valves, can be determined simultaneously. The rates of flow can be summed through both apply and release valves. The summed flow rates can be converted to a volume reading. The predetermined corresponding pressure value can be compared to the command pressure value to determine if a difference exists. From an existing difference it is determined which of the apply and release valves must be actuated to eliminate the difference. At least one of the apply and release valves are controlled to eliminate the error difference and produce a steady state.
At least one of the apply and release valves can be oscillated by applying a dither feature to the command pressure value, causing the value to oscillate between upper and lower values between which the steady state is centered.
Estimation of caliper pressure can further include comparing line in solenoid valve flow to a lookup table of apply pressure values in a memory of a control module of the system, comparing line out solenoid valve flow to a lookup table of release pressure values in a memory of a and control module of the system and summing the pressure values together to obtain estimated caliper pressure. A saturation point restriction based on system limits can be applied to the estimated caliper pressure. All variables can be placed under a saturation point restriction based on system limitations. All variable values can be reset to 0 upon brake pedal release. Command pressure and accumulator pressure can actualize upon logic initialization. Release of the brake pedal can be recognized by the logic; ending the process. The dither feature can be turned off below a predefined command pressure value.
Another aspect of the present invention in an electronically controlled vehicular braking system incorporating a caliper actuation assembly including at least one valve, provides a method including the steps of generating pressure application command to the caliper actuation assembly by controlling positioning of the at least one valve thereof, estimating the actual pressure applied from a lookup table of predefined levels of applied pressure corresponding to valve position and generating periodic modulated valve control signals until a near zero difference is determined to exist between the command and estimated applied pressures from the predetermined lookup table data correlating estimated applied pressure and valve position.
Another aspect of the present invention provides an electronically controlled brake system including a controller for controlling pressure applied by a brake caliper through adjustment of at least one valve associated with the caliper, required valve adjustment being determined from a lookup table of estimated pressure applied correlated to valve position, with valve position being adjusted continuously until the estimated pressure applied is substantially identical to pressure required to produce a brake apply of required level as it pertains to brake command pressure.
Another aspect of the present invention provides a brake control system including means for determining a command caliper pressure, means for determining an estimated caliper pressure, means for comparing the command to the estimated caliper pressure to produce an error value means for multiplying the error value by a predetermined gain to produce a valve command and means for controlling apply and release valves responsive to the valve command.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.